Extended-depth-of-field imaging with an ultra-thin folded lens.

Optical systems with extended depth of field (EDOF) are crucial for observation and measurement applications, where achieving compactness and a substantial depth of field (DOF) presents a considerable challenge with conventional optical elements. In this paper, we propose an innovative solution for the miniaturization of EDOF imaging systems by introducing an ultra-thin annular folded lens (AFL). To validate the practical feasibility of the theory, we design an annular four-folded lens with an effective focal length of 80.91 mm and a total thickness of only 8.50 mm. Simulation results show that the proposed folded lens has a DOF of 380.55 m. We further developed an AFL-based test system exhibiting a resolution of 0.11 mrad across a wide wavelength range of 486-656 nm. Additionally, we present experimental results from a miniature compact prototype, which further highlights the promising potential of folded lenses for long-range EDOF imaging.

Curved fiber compound eye camera inspired by the Strepsiptera vision.

The Strepsiptera vision possesses intriguing features of a large field of view (FOV) and relatively high resolution compared to normal compound eyes. However, it presents a significant challenge of the mismatch between the curved compound eyelet lens array and the planar image sensor to image in a large FOV for artificial compound eyes (ACE). We propose what we believe to be a novel curved fiber compound eye camera (CFCEC) here, which employs coherent fiber bundles as the optical relay system to transmit sub-images curvilinearly. A total of 106 eyelets are arranged based on a scheme similar to the Goldberg polyhedron, with the advantages of uniform interval and minor edge blindness. Then, a prototype of the CFCEC is fabricated and assembled. A series of experiments are conducted to assess the FOV, contrast, resolution, and overlap rate of FOV of the prototype. The results prove that the CFCEC has a total FOV of up to 160°×160° and a total overlap rate of FOV of approximately 65%, demonstrating the promising potential of the CFCEC in various applications, such as panoramic surveillance, 3D detection, and motion tracking.

High sensitivity composite F-P cavity fiber optic sensor based on MEMS for temperature and salinity measurement of seawater.

We proposed an optical fiber salinity sensor with a composite Fabry-Perot (F-P) cavity structure for simultaneous measurement of temperature and salinity based on microelectromechanical system (MEMS) technology. The sensor contains two sensing cavities. The silicon cavity is used for temperature sensing, and the seawater cavity processed by the glass microstructure is sensitive to the refractive index of seawater for salinity sensing. At the same time, the influence of the salinity-temperature cross-sensitivity error of the seawater cavity is effectively compensated by using the temperature single parameter sensitivity characteristics of the silicon cavity. The structural design of the sensor seawater cavity includes a cross-shaped groove and a cylindrical fluid cavity. The surface hydrophilicity treatment was performed on the interior of the cavity to solve the effect of no water injection in the cavity caused by the miniaturization of the sensor. The optical path difference (OPD) demodulation method is used to demodulate the two F-P cavities with large dynamic range and high resolution. In the range of 5∼40°C and 5∼ 40 ‰, the temperature and salinity sensitivity of the sensor can reach 110.25 nm/°C and 178.75 nm/‰, respectively, and the resolution can reach 5.02 × 10-3°C and 0.0138‰. It has the advantages of mass production, high stability, and small size, which give it great potential for marine applications.

High Sensitivity Temperature Sensing of Long-Period Fiber Grating for the Ocean.

In this study, a new temperature sensor with high sensitivity was achieved by four-layer Ge and B co-doped long-period fiber grating (LPFG) based on the mode coupling principle. By analyzing the mode conversion, the influence of the surrounding refractive index (SRI), the thickness and the refractive index of the film on the sensitivity of the sensor is studied. When 10 nm-thick titanium dioxide (TiO2) film is coated on the surface of the bare LPFG, the refractive index sensitivity of the sensor can be initially improved. Packaging PC452 UV-curable adhesive with a high-thermoluminescence coefficient for temperature sensitization can realize high-sensitivity temperature sensing and meet the requirements of ocean temperature detection. Finally, the effects of salt and protein attachment on the sensitivity are analyzed, which provides a reference for the subsequent application. The sensitivity of 3.8 nm/°C in the range of 5-30 °C was achieved for this new sensor, and the resolution is about 0.00026 °C, which is over 20 times higher than ordinary temperature sensors. This new sensor meets the accuracy and range of general ocean temperature measurements and could be used in various marine monitoring and environmental protection applications.

Optical electrocardiogram monitor with a real-time analysis of an abnormal heart rhythm for home-based medical alerts.

Sudden cardiac death (SCD) caused by cardiovascular disease is the greatest hidden danger to human life, accounting for about 25% of the total deaths in the world. Due to the early concealment of SCD and the heavy medical burden of long-term examination, telemedicine combined with home monitoring has become a potential medical alert method. Among all the existing human cardiac and electrophysiology monitoring methods, optics-based sensors attract the widest attention due to the advantages of low delay, real-time monitoring, and high signal-to-noise ratio. In this paper, we propose an optical sensor with the capabilities of long-term monitoring and real-time analysis. Combining an R-peak recognition algorithm, Lorenz plots (LP), and statistical analysis, we carried out the consistency analysis and result visualization of ECG sequences over 1 h. The results of 10 subjects show that the R-peak recognition accuracy of the optical ECG monitor is higher than 97.99%. The optical system can display abnormal heart rhythm in real-time through LP, and the readability is good, which makes the system suitable for self-monitoring at home. In addition, this paper provides a detailed long-term monitoring assessment method to effectively guide the practical clinical transformation of other optical wearable devices.

Ratiometric Optical Fiber Dissolved Oxygen Sensor Based on Fluorescence Quenching Principle.

In this study, a ratiometric optical fiber dissolved oxygen sensor based on dynamic quenching of fluorescence from a ruthenium complex is reported. Tris(4,7-diphenyl-1,10-phenanthrolin) ruthenium(II) dichloride complex (Ru(dpp)32+) is used as an oxygen-sensitive dye, and semiconductor nanomaterial CdSe/ZnS quantum dots (QDs) are used as a reference dye by mixing the two substances and coating it on the plastic optical fiber end to form a composite sensitive film. The linear relationship between the relative fluorescence intensity of the ruthenium complex and the oxygen concentration is described using the Stern-Volmer equation, and the ruthenium complex doping concentration in the sol-gel film is tuned. The sensor is tested in gaseous oxygen and aqueous solution. The experimental results indicate that the measurement of dissolved oxygen has a lower sensitivity in an aqueous environment than in a gaseous environment. This is due to the uneven distribution of oxygen in aqueous solution and the low solubility of oxygen in water, which results in a small contact area between the ruthenium complex and oxygen in solution, leading to a less-severe fluorescence quenching effect than that in gaseous oxygen. In detecting dissolved oxygen, the sensor has a good linear Stern-Volmer calibration plot from 0 to 18.25 mg/L, the linearity can reach 99.62%, and the sensitivity can reach 0.0310/[O2] unit. The salinity stability, repeatability, and temperature characteristics of the sensor are characterized. The dissolved oxygen sensor investigated in this research could be used in various marine monitoring and environmental protection applications.

Recent Development of Tunable Optical Devices Based on Liquid.

Liquid opens up a new stage of device tunability and gradually replaced solid-state devices and mechanical tuning. It optimizes the control method and improves the dynamic range of many optical devices, exhibiting several attractive features, such as rapid prototyping, miniaturization, easy integration and low power consumption. The advantage makes optical devices widely used in imaging, optical control, telecommunications, autopilot and lab-on-a-chip. Here, we review the tunable liquid devices, including isotropic liquid and anisotropic liquid crystal devices. Due to the unique characteristics of the two types of liquids, the tuning principles and tuning methods are distinguished and demonstrated in detail firstly and then some recent progress in this field, covering the adaptive lens, beam controller, beam filter, bending waveguide, iris, resonator and display devices. Finally, the limitations and future perspectives of the current liquid devices are discussed.

Tunable multi-wavelength optofluidic Dammann grating with beam splitting property.

Dammann grating (DG) is a binary beam splitter. Traditional DG is pure solid and cannot be modulated for different working wavelength. We report a tunable multi-wavelength DG based on a liquid-solid hybrid structure. Two glass plates are bonded by UV adhesive strips, one has a periodic grooves structure made by photoresist, the other has two drilled holes as inlet and outlet, respectively. A microfluidic mixer connected the inlet mixes of two miscible liquids with different flow rates to adjust the refractive index of the mixed liquid entering DG from 1.351 to 1.473. In the experiment, the real-time tunability has shown the DG achieves well beam splitting effect when parameter N is integer, 7 × 7 light spots are arranged in order with good uniformity. For λ = 632.8 nm, spot size uniformity is about 78.38% and power uniformity is ∼71.01%. For λ = 532 nm, the spot size and power uniformity are about 77.17% and 64.32%, respectively. The experiment also demonstrates this DG's suitability for near-infrared light. This work is the first study of tunable DG based on liquid-solid hybrid structure and possesses special merits as compared to its solid counterpart, such as simple fabrication, tunability and multi-wavelength applicability, which make it have an extensive prospect in optofluidic networks and optical devices.

Design and characteristics of a Maxwell force-driven liquid lens.

Varifocal lenses (especially large-aperture lenses), which are formed by two immiscible liquids based on electrowetting and dielectrophoretic effects, are usually modulated by an external high-voltage power source, with respect to the volume of the liquid. Hence, a Maxwell force-driven liquid lens with large aperture and low threshold voltage is proposed. With the polarization effect, the accumulated negative charges on the surface of the polyvinyl chloride/dibutyl adipate gel near the anode results in the generation of Maxwell force and deformation with cosine wave. The effect of surface roughness on wettability is linear with the cosine of the contact angle, leading to a sharp reduction in the threshold voltage when the volume of liquid is increased. When the volume of the droplet increases to 80 µl, the threshold voltage is about 10 V. Hence, the aperture of polarization effect-driven liquid lenses can potentially reach the centimeter level. Moreover, when Maxwell force increases, the lens ranges from concave to convex lens, which holds great promise in rich application such as those in light-sheet microscopes and virtual reality systems.

Design and characteristics of tunable in-plane optofluidic lens actuated by viscous force.

In this Letter, we report a tunable in-plane optofluidic lens based on a new regulation method. The viscous force (VF) adjusts a 68# white mineral oil-air interface and focal length (f). Two glass plates bonded by ultraviolet adhesive strips form a lens chamber. Liquid enters the chamber by capillary action and forms a convex interface due to VF. As the liquid filling amount increases, VF is enhanced, and the interface deforms. Because of the uneven VF, interface is aspheric, which can reduce the lens aberration. Bendings on both sides of the interface caused by edge effect lead to an even polynomial profile of the entire interface, and they can be used for aberration correction of an in-plane spherical reflector. Experiments demonstrate the continuous tuning of f from 17.7 to 45.1 mm. The positive longitudinal spherical aberration (LSA) is effectively suppressed below 0.078 when f<35.5mm. Interface with a large negative LSA is used for spherical reflector aberration correction. Simulation results proved that the light spot improvement rate is>90%, and the maximum reached 99%.

Liquid Lens with Large Focal Length Tunability Fabricated in a Polyvinyl Chloride/Dibutyl Phthalate Gel Tube.

Usually, an adaptive liquid lens only has a positive focal length, which severely limits its application in imaging and other fields. Therefore, a liquid lens consisting of polyvinyl chloride/dibutyl phthalate (PVC/DBP) gel, glycerol solution, and a glass substrate is proposed to extend the dynamic focal length range. A spherical tube is formed by the PVC/DBP gel under the effect of hydrostatic and surface tensions, which is used to restrict the glycerol solution. The PVC/DBP gel does not deform under the effect of an electric field, so the tangent line at the three-phase junction changes with the change of contact angle, which leads to an enlargement of the dynamic focal length range. At different voltage values, the proposed lens can be configured to work in three different schemes, namely, converging light, nondeflecting light, and diverging light. Here, the proposed lens has high imaging quality; the resolution is better than 114 lp/mm. A lens with a reconfigurable focal length holds great promise in diverse applications such as fluorescence detection, beam shaping, and adaptive optics.

Measurement of cloud particles in a cloud chamber based on interference technology.

Based on interference technology, a cloud particle measurement system is designed. The scattering angle of the system is selected as 90°. The iterative mean filter algorithm is modified, and the system testing using laboratory measurement is completed. The measurement of the spectral distribution of warm cloud particles in a cloud chamber is realized. Similar particle-sized distributions are observed under different pressures, and the particle size is mainly distributed in the range of 5 to 50 µm. The peak appears at particle sizes of 20 to 30 µm. This system features potential applications in cloud microphysics research.

Determination of the orientation of transparent spheroids using interference technology.

The distribution of light scattered by a spheroidal particle is sensitive to the orientation of the particle. Based on interference technology, we present a method for obtaining the orientation of the spheroidal particle from in-focus and out-of-focus images. We simulate the in-focus images using the LightTools. From the optical transfer matrix theory, we obtain both the in-focus and corresponding out-of-focus images using Matlab. We find that the glare-point distribution of the in-focus image exhibits axial symmetry for a spheroidal particle and that it is perpendicular to the speckle orientation of the out-of-focus image. We establish an interferometric particle imaging system to experimentally acquire the out-of-focus and in-focus images of the transparent spheroidal particles. The experimental results agree with the simulations. We are thus able to propose a method for obtaining the orientation of a spheroidal particle using either the in-focus image or out-of-focus image. The method has potential applications in particle measurements.

Multidispersed bubble-size measurements by interferometric particle imaging at scattering angles of 90° and 45°.

Measurements of multidispersed bubble diameter are important in hydraulics, biology, and other such fields. Interferometric particle imaging (IPI) systems are used to measure the bubble diameter. In geometric approximation, the scattering angle cannot be greater than 2arccosm. We propose a universal method for calculating the bubble diameter at wide scattering angles and use an IPI system to measure the bubble-size distribution at 90° and 45°. The particle size distributions measured at two angles are consistent with each other. The results of this study indicate that this method is suitable for measuring the bubble-size distribution.

Highly sensitive fiber-optic chemical pH sensor based on surface modification of optical fiber with ZnCdSe/ZnS quantum dots.

The pH value plays a vital role in many biological and chemical reactions. In this work, the fiber-optic chemical pH sensors were fabricated based on carboxyl ZnCdSe/ZnS quantum dots (QDs) and tapered optical fiber. The photoluminescence (PL) intensity of QDs is pH-dependence because protonation and deprotonation can affect the process of electron-hole recombination. The evanescent wave of tapered optical fiber was used as excitation source in the process of PL. To obtain higher sensitivity, the end faces of fiber were optimized for cone region. By lengthening the cone region and shrinking the end diameter of optical fiber, evanescent wave was enhanced and the excitation times of QDs were increased, which improved the PL intensity and the sensitivity of the sensor. The sensitivity of sensor can reach as high as 0.139/pH in the range of pH 6.00-9.01. The surface functional modification was adopted to prepare sensing films. The carboxyl groups on the QDs ligands are chemically bonded to the fiber surface, which is good for response time (40 s) and stability (decreased 0.9 % for 5 min). These results demonstrated that ZnCdSe/ZnS QDs-based fiber-optic chemical pH sensors are promising approach in rapid and precise pH detection.

Relationship between visual acuity and OCT angiography parameters in diabetic retinopathy eyes after treatment.

PURPOSE: To assess the relationship between visual acuity and OCT angiography parameters in diabetic retinopathy eyes after treatment, and to analyze the relative factors in PDR eyes. METHODS: A total of 89 eyes, including 42 eyes with non-PDR (NPDR), and 47 eyes after vitrectomy with PDR were included and underwent OCTA. All images were processed by Python or FIJI. Multivariable linear regression models were used to analyze the associations between postoperative BCVA and OCTA parameters in PDR patients. RESULTS: Postoperative OCTA parameters including deep capillary plexus (DCP) parafoveal and perifoveal vessel density (VD), DCP parafoveal and perifoveal vessel length density (VLD), DCP fractal dimension (FD), choriocapillaris plexus (CCP) VD, CCP VLD, were significantly lower in the PDR group than in the NPDR group. In the superficial capillary plexus (SCP), we found a negative correlation between the postoperative BCVA and VD (parafovea: ß coefficient = -0.351, p = 0.023; perifovea: ß coefficient = -0.338, p = 0.036). Perifoveal VLD (ß coefficient = -0.343, p = 0.031) and FD (ß coefficient = -0.375, p = 0.016) of the SCP were also negatively correlated with postoperative BCVA. Regarding the DCP, perifoveal VD (ß coefficient = -0.396, p = 0.008), perifoveal VLD (ß coefficient = -0.334, p = 0.025), vessel tortuosity (VT) (ß coefficient = -0.369, p = 0.015) were negatively correlated with postoperative BCVA. In CCP, VLD (ß coefficient = -0.373, p = 0.023) and number of flow voids (ß coefficient = -0.334, p = 0.036) exhibited a negative association with postoperative BCVA. CONCLUSIONS: Postoperative BCVA of PDR patients was related to OCTA parameters of the SCP (parafoveal and perifoveal VD, perifoveal VLD and FD), DCP (perifoveal VD, VLD, and VT) and CCP (VLD and number of flow voids).

Self-adaptive demodulation for polarization extinction ratio in distributed polarization coupling.

A self-adaptive method for distributed polarization extinction ratio (PER) demodulation is demonstrated. It is characterized by dynamic PER threshold coupling intensity (TCI) and nonuniform PER iteration step length (ISL). Based on the preset PER calculation accuracy and original distribution coupling intensity, TCI and ISL can be made self-adaptive to determine contributing coupling points inside the polarizing devices. Distributed PER is calculated by accumulating those coupling points automatically and selectively. Two different kinds of polarization-maintaining fibers are tested, and PERs are obtained after merely 3-5 iterations using the proposed method. Comparison experiments with Thorlabs commercial instrument are also conducted, and results show high consistency. In addition, the optimum preset PER calculation accuracy of 0.05 dB is obtained through many repeated experiments.

2D shape reconstruction of submillimetric irregular rough particles from speckle pattern in interferometric particle imaging measurement.

Particle shape is a significant feature of irregular particles. The interferometric particle imaging (IPI) technique has been introduced to retrieve submillimetric irregular rough particle shapes, while inevitable experimental noises hinder the convergence of two-dimensional (2D) particle shapes from single speckle patterns. In this work, a hybrid input-output algorithm with shrink-wrap support and oversampling smoothness constraints is utilized to suppress the Poisson noise in IPI measurement and recover accurate 2D shapes of particles. Our method is tested in numerical simulations on ice crystal shapes and actual IPI measurements on four different types of irregular, rough particles. The shape similarity of the reconstructed 2D shape has reached an average Jaccard Index score of 0.927, and the relative deviation of the reconstructed size is within 7% for all 60 tested irregular particles at the maximum shot noise level of 7.4%. Furthermore, our method has obviously reduced the uncertainty in the 3D shape reconstruction of irregular, rough particles.

Influence of vibration disturbance during polarization coupling measurement of polarization-maintaining fiber.

The principle of the mode cross coupling in polarization-maintaining fiber based on white-light interferometry was analyzed. The method of measuring the polarization mode coupling with a spatial Michelson interferometer was presented. Analysis and emulation were carried out for the vibration disturbance signal caused by the mechanical scanning and the influence the vibration imposed on the judgment of coupling intensity. The interference signal envelope is extracted by a Hilbert transform and fitted by a Gaussian least-squares method under the different scanning speed. It is indicated that the detection accuracy varies with the vibration amplitude, which varies with scanning speed. The best scanning speed of the system should be from 0.7 mm/s to 0.9 mm/s to achieve the minimum detection error.

[Research on trace gas spectral measurement on intra-cavity fiber optic laser].

Due to the advantages of immunity of electrical/electronic, high performance cost ratio, remote detection and multiplexing capability, intra-cavity fiber optic gas measurement has aroused wide concern. The trace gas measurement system has been developed based on the elaborated gas cell and reflector. The wavelength sweeping technique (WST) is realized when the Fabry-Perot type tunable optical filter is applied by the sawtooth driver voltage. Multi absorption lines can be obtained and one scanning measurement with WST is equal to multiple independent detections, so the gas measurement sensitivity is improved remarkably. The experimental results show that the acetylene detection sensitivity is reduced to less than 100 ppm and the relative measurement error is less than 3% of practical gas concentration.